Oscillators with automatic frequency and phase control (AFPC) are used, for example, in the implementation of synchronous video detectors. It is commonplace practice to construct a synchronous video detector within the confines of the same monolithic integrated circuit (IC) as a plural-stage video intermediate-frequency (IF) amplifier. The synchronous detector is commonly of a "tree" type in which the collector currents of a first pair of emitter-coupled transistors operated as a differential-mode transconductance amplifier are synchronously switched by respective further pairs of emitter-coupled transistors operated as single-pole-double-throw current-mode switches. The "tree" type of synchronous detector can be designed to have appreciable conversion gain (20 dB or so), reducing the need for video IF voltage gain in the picture (PIX) IF amplifier supplying amplified video IF signal to the base electrodes of the first pair of emitter-coupled differential-amplifier transistors.
The recent practice in regard to implementing the IF filtering and gain functions in television signal receiving apparatus is to use a lumped or "block" filter preceding a gain-block amplifier comprising plural stages arranged in direct-coupled cascade within the confines of a monolithic integrated circuit (IC). The "block" filter is commonly a surface-acoustic-wave (SAW) filter used to provide the entire passband shape and adjacent channel attenuation required by a television receiver. No interstage tuning is used in the gain-block IC amplifier. The amplified IF signal from the gain-block IC amplifier is subsequently detected substantially within the confines of the monolithic integrated circuit, to emerge from the IC as baseband composite video signal and as sound IF at 4.5 MHz. The amplified IF signal is filtered from these signals to reduce the likelihood of regeneration causing the gain-block amplifier to oscillate. Additional information on SAW filters and on block filtering and amplification may be found, for example, in Chapter 13 of the book TELEVISION ENGINEERING HANDBOOK; K. Blair Benson, Editor in Chief; McGraw-Hill Book Company, New York; 1986. U.S. patent application Ser. No. 08/080,705 filed 24 Jun. 1993 by Jack Rudolph Harford, entitled PARALLEL UNTUNED VIDEO IF AMPLIFIERS SUPPLIED SIGNALS FROM TV 1.sup.ST DETECTOR VIA RESPECTIVE INPUT FILTERS, and incorporated herein by reference provides further information concerning the use of SAW filters and gain block amplifiers for intermediate frequencies in television receivers.
Prior-art ICs having video IF amplifiers in gain-block configurations followed by synchronous detectors have used controlled oscillators of the following general type. An LC tank circuit serves as as a collector load for a transistor overdriven by regenerative feedback to its base electrode, so as to switch between saturated conduction and cut-off. This common arrangement exhibits some problems, however.
The voltage signal across the LC tank circuit is typically a few volts peak-to-peak, so stray coupling to the input of the video IF amplifier is apt to be consequential. This presents problems especially when IF sensitivity is made greater in response to commercial requirements.
The LC tank circuit used as the collector load for the switched transistor has a voltage signal across it from which harmonic content is removed by filtering, but the switched transistor current flow is conducted in unbalanced form through the IC "ground" connection and has appreciable harmonic content. The "ground" bus on the printed-circuit board on which the IC is mounted radiates this harmonic content back to the tuner to cause problems with Channel-8 reception, etc. These problems persist even when the LC tank circuit is driven push-pull by a pair of transistors switched in antiphase, since there is appreciable common-mode harmonic content.
The inventor sought an oscillator in which the LC tank circuit is in a portion of the circuitry where signal levels are at their lowest, rather than at their highest, to minimize the problems of radiation back to the tuner and to the IF amplifier input port. In order to reduce the oscillations and harmonics thereof appearing on the operating supply busses for the IC, the inventor sought an oscillator of balanced construction. A differential ampitier of the so-called "long-tailed pair" type using a pair of emitter-coupled bipolar transistors in its construction is a favored circuit with balanced construction suitable for inclusion in a monolithic IC. Locating the LC tank circuit between the base electrodes of the emitter-coupled transistors, rather than between their collector electrodes, allows the low-resistance path through the tank coil to keep the base electrodes at substantially equal direct potentials, simplifying input biasing.